The subject matter described herein relates generally to utility grid support and, more particularly, to methods and apparatus for facilitating control of a fault-induced delayed voltage recovery (FIDVR) on a utility grid using photovoltaic devices.
Many known electric utility grids include a plurality of interconnected known transmission and distribution (T&D) systems. Many of these known T&D systems include a plurality of interconnected regions that are geographically defined by T&D system features, for example, substation locations. In at least some known T&D regions, a significant concentration of known induction motors are present. Many of these known induction motors have constant-torque features and low-inertia characteristics. Examples of such constant-torque, low-inertia induction motors include residential and commercial air-conditioner (A/C) compressor motors. Moreover, many of these known residential A/C compressor motors are commercially sold without undervoltage (UV) protection. Significant penetration of such known induction motors into residential neighborhood and commercial regions of local T&D systems at least partially determine a T&D system's vulnerability to a fault-induced delayed voltage recovery (FIDVR) event.
FIDVR events are cascading events that are initiated by an electrical fault occurring on at least one portion of the T&D system. Such electrical faults typically automatically initiate fault-clearing features of the T&D system that quickly isolate the fault within approximately three cycles, however, the voltage of a region of the T&D system may remain at a significantly reduced level for several seconds after the fault has been cleared. The extended period of voltage depression is typically caused by high concentrations of induction motor loads with constant torque and low inertia that begin to slow down and have flux collapse substantially simultaneously with the voltage reduction and may slow down sufficiently to stall under their associated loading. These induction motors are sometimes referred to as “stall-prone” induction motors and the stalled condition is sometimes referred to as a “locked-rotor” condition. As these induction motors slow down, they draw increased reactive power from the T&D system. Moreover, such stalled induction motors require approximately 5 to 6 times their steady-state operating current during locked-rotor conditions. However, the increased current at low voltage conditions may not unstall the motor, that is, the rotor will not be released from the locked-rotor condition.
The heavy locked-rotor current demands on the T&D system result in T&D system voltage remaining significantly depressed for a period of time, typically a few seconds, after the fault is cleared, thereby leading to a first cascading effect, i.e., a cascading voltage collapse through adjacent portions of the interconnected T&D system that may extend further through the utility power grid.
A second cascading effect includes a response to the real and reactive power demands on the electric power generators coupled to that portion of the T&D system. If voltages stay depressed long enough, the associated generators trip or, alternatively, over-excitation limiting devices limit and/or reduce reactive power generation, thereby facilitating further voltage reductions and a possible system-wide voltage collapse.
A third cascading effect includes the stalled induction motors drawing the increased current such that they are removed from service by thermal protection devices with an inverse time-current characteristic that is usually set for 3-20 seconds. The combined effect of larger induction motors and smaller induction motors tripping over such a short period of time may result in significant load loss, as can the loss of generation described above, with a potential effect of a voltage recovery overshoot inducing a high-voltage condition. Depending on the size of the region affected, the associated load reduction may extend from a few kilowatts (kW) up to hundreds of megawatts (MW).
At least some known T&D systems may be configured to receive retrofit protection systems that facilitate clearing faults more quickly, however, FIDVR events that are initiated within as little as 3 cycles will not be prevented. Also, at least some known T&D systems may be configured to receive installed reactive power sources, for example, large capacitor banks. Further, at least some known T&D systems may be resectionalized to further limit faults to smaller portions of the affected T&D system. However, these two potential solutions require extended time periods to design, construct, and install and in general, may not be sufficient to mitigate FIDVR events. They also require large physical footprints, significant capital investment, and long-term operational and maintenance costs. Moreover, at least some known T&D systems may be configured to receive retrofit UV load shedding schemes to trip stall-prone loads as soon as possible once the fault condition is detected, however, such load shedding schemes typically require at least partial power outages to some portions of the T&D system. Another potential long-term solution includes promotion of unit-level replacements of existing stall-prone A/C units with A/C units that include UV protection. This solution may take decades to implement and may encounter significant public reluctance to pay more for residential A/C units, thereby extending the time horizon for substantial implementation.